Dynamic Compressive Behavior and Internal Damage Evolution of Polypropylene Fiber-Reinforced Alkali-Activated Slag Geopolymer Mortar

Ordinary Portland cement (OPC) production is energy- and carbon-intensive, which has motivated the development of more sustainable alternatives such as alkali-activated materials, including granulated blast furnace slag (GBFS) geopolymer mortar. Geopolymer mortar, however, is brittle, has low tensile strength and poor crack resistance, and its damage evolution under dynamic impact remains insufficiently understood. Here we show that polypropylene fiber (PPF) reinforcement substantially improves the dynamic impact resistance of GBFS-based geopolymer mortar with only a limited effect on static compressive strength. Mortars with PPF volume contents of 0%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0% were prepared at a water-to-binder ratio of 0.55. Flowability, static compressive strength, and dynamic mechanical properties were evaluated using a split Hopkinson pressure bar at strain rates of 50 s−1–130 s−1, and failure morphology was examined through SEM and CT analysis. Increasing PPF content reduced flowability from 253 mm to 224 mm at 1.0% but had little effect on static strength. Dynamic compressive strength, dynamic increase factor (DIF), and ultimate toughness increased with both strain rate and fiber content. The 1.0% PPF mortar reached a maximum DIF of 2.129 and a 99.3% increase in impact toughness. CT analysis further showed that PPF suppressed the formation of large internal voids: approximately 31, 2, and 5 voids larger than 4 mm3 were detected in the 0.6%, 0.8%, and 1.0% PPF specimens, respectively. The improvement is attributed to fiber bridging and pull-out, which consume impact energy and inhibit crack initiation and propagation. These results indicate that, within the tested range, a PPF content of 1.0% is effective for impact-resistant geopolymer mortar.